1. Field of the Invention
The present invention relates generally to a handover method in a mobile communication system, and in particular, to a cell search method for handover between an asynchronous mobile communication system's base station (or asynchronous base station) and a synchronous mobile communication system's base station (or synchronous base station).
2. Description of the Related Art
3rd generation (3G) mobile communication systems employ code division multiple access (CDMA) technology, and provide consistent service capable of transmitting packet-based text, digitized voice and video, and multimedia data at a high rate of over 2 Mbps no matter where a user of a mobile phone or terminal is in the world. Such 3G mobile communication systems are classified as either universal mobile telecommunication service (UMTS) adopted as the European standard or international mobile telecommunications-2000 (IMT-2000), adopted as the U.S. standard.
UMTS is an asynchronous mobile communication system employing CDMA modulation based on global system for mobile communication (GSM) and general packet radio services (GPRS) technologies which are extensively used in Europe and other regions. IMT-2000 is a synchronous mobile communication system based on a 2nd generation (2G) CDMA system such as IS-95 and J-STD008, which is used in South Korea, the United States, and Japan. The two systems are moving toward harmonization, and accordingly, research is being conducted on several technologies that offers compatibility between the two systems. One important field of research is directed towards technology related to handover which may occur while a mobile station travels between the two systems.
Handover is the term used for technology that enables a user to perform communications without dropping calls, even though a mobile station travels from one cell to another in a cellular mobile communication system. Handover is classified into two types: soft handover and hard handover. In a soft handover situation, the mobile user is performing communications while using a plurality of channels in an area where two or more cells overlap with each other. If the quality of a particular channel decreases below a predetermined threshold, one of the channels, the corresponding channel, is dropped. In a hard handover situation, the channel to the previous cell (i.e., the cell the mobile user is moving away from) is dropped and then access to a neighbor cell is attempted as the mobile station moves between cells.
A mobile station undergoing communication measures information on neighbor cells and reports the measured information to a base station in at least two situations: first, if a level of a received pilot signal is lower than or equal to a threshold; and, second, at the request of the base station. The operation of measuring information on neighbor cells is called “cell searching.” The reported information is used to determine when handover should occur. During standby, handover is automatically performed in a mobile station without direction of the system. During a call, a handover direction is transmitted to the mobile station over a traffic channel in the case of a synchronous mobile communication system, and over a dedicated channel in the case of an asynchronous mobile communication system.
When a dual-mode mobile station moves from an asynchronous mobile communication system to a synchronous mobile communication system, hard handover occurs. This is because it is impossible for the mobile station to simultaneously connect channels to the two different systems. When hard handover is performed, communication between a mobile station and an asynchronous mobile communication system is discontinued for a time when the mobile station is searching information on neighbor cells.
In a mobile communication system employing CDMA technology, a terminal performs cell search during idle-state handover or active-state handover. In a synchronous CDMA system, all base stations are synchronized using a global positioning service (GPS) satellite. Each base station can then inform the mobile stations of pseudo-random noise (PN) offset information for a pilot from its neighbor cell. This enables the mobile stations to perform neighbor cell search at a time when the corresponding PN offset has elapsed from a reference time. Knowing the PN offset time allows the mobile station to complete the neighbor cell search in a short period of time. Cell search during initial power on requires a long time, however, since time alignment with the base station is necessary.
When a mobile station moves from a service area of an asynchronous mobile communication system to a service area of a synchronous mobile communication system, the mobile station must perform all the same procedures as those performed during the initial cell search. This is because the mobile station does not have timing information for a synchronous base station. During cell search for a synchronous base station (hereinafter referred to as “CDMA cell search”), a mobile station first searches a pilot signal. Thereafter, the mobile station decodes a message on a synchronization channel to acquire information (e.g., PN offset and system time information), so that the mobile station can communicate with the synchronous base station, and then matches its synchronization to the synchronous base station.
The synchronization information message on a synchronization channel is written in a synchronization channel frame transmitted from a synchronous base station. Since the synchronization channel has 96 transmission bits per 80 ms frame, and a synchronization signal message including PN offset information is 221 bits in length, a mobile station requires at least 240 ms (it takes 2 full frames (2×96=198)+23 bits of a third frame, to transmit all 221 bits; therefore, 3 frame transmission time periods are needed. 80 ms×3=240 ms) for a message analysis time. In addition, when a reception start point is not identical to a message start point, the mobile station needs to interrupt communication for about 513.3 ms. This includes the time it waits for a start point of the synchronization signal message, even though it is assumed that a received frame has no error. An increase in the cell search time may cause a loss of communication data between an asynchronous mobile communication system and a mobile station.
For the same reason, a mobile station in communication with an asynchronous mobile communication system cannot perform a CDMA cell search, simply with a compressed mode for monitoring other frequency bands for an idle period of a frame, secured by data compression. This is because a CDMA cell search generally requires a long search time, and cannot be completed in the relatively short idle period (usually shorter than 10 ms) of a frame.